A differential amplifier including two active elements such as transistors and field effect transistors (FET) are generally utilized in an first (input) stage of an operational amplifier incorporated in an integrated circuit (IC). In such a differential amplifier, a variable resistor is interposed via a fixed resistor between a control electrode such as a base terminal of a transistor and ground for controlling the gain of the differential amplifier. The control electrode is connected via another fixed resistor to a power supply so that two fixed resistors and the variable resistor function as a voltage divider wherein the voltage at the control electrode is variable in accordance with the change of the resistance of the variable resistor. As the variable resistor a rotational type or a slidable type is usually employed. The variable resistor is operated by an operator or a user of the amplifier in order to control the gain of the amplifier, by means of changing the position of the movable (rotatable or slidable ) contact thereof.
Assuming the resistance of the first fixed resistor which is interposed between the power supply and the control electrode Ra, the resistance of the second resistor interposed between the control electrode and one terminal of the variable resistor Rb and the resistance between the terminal and a movable contact of the variable resistor Rc, the voltage Vb at the control electrode is obtained by the following equation: ##EQU1## WHEREIN Vs is a voltage of the power supply.
Usually the ratio of the first and second resistances Ra and Rb has .+-.1.about..+-.5% variation, if these fixed resistors are made on an IC chip. Therefore, in order to obtain a predetermined voltage at the control electrode by setting the movable contact of the variable resistor at a predetermined position, the maximum resistance of the variable resistor must be selected so as to compensate for the variations of the ratio between the fixed resistors. Further in order to obtain a predetermined gain by a differential amplifier, the voltage at the control electrode of the other active element should be at a predetermined voltage. When all of these requirements are fulfilled, the gain of the differential amplifier exactly corresponds to the position of the movable contact of the variable resistor.
However, variable resistors usually have a considerable variation in maximum resistances because of mass production. The maximum resistances of variable resistors vary within as much as .+-.30% of the rated resistance. Therefore, the deviation of the bias voltage at the control electrode from a predetermined value which corresponds to a predetermined position of the movable contact is apt to be undesirably large. This means even though the resistances of the fixed resistors are within narrow variation limits, any given gain obtained by a conventional type of gain control circuit in combination with a differential amplifier does not correspond to any one predetermined position of the movable contact of the variable resistor.
Because of the above mentioned reason it has, until now, been extremely difficult to construct a practical gain control circuit for differential amplifiers in which the amplifier to amplifier variation of the gain, obtained by setting the variable resistor in any one set or predetermined position, is negligible.